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    Welflow is a combined company of four factories whose products are complementary to each other. In addition to the mainland China market, the products are exported to South Korea, Taiwan(China), the United States, Singapore, Malaysia, Australia, and other countries and regions.
  • 2020-02-08

The flange is a device for connecting pipes or instruments and plays the role of connection and fixing.

  • A common flange is a round steel form.
  • The shape is divided into plate flange, blind flange, butt-welding flange, Slip-on flange, etc.
  • Production methods are divided into forging and casting
  • Material is mostly stainless steel or carbon steel

The flange is widely used worldwide and the amount is very large, so it has formed a number of regional system standards, which are generally based on the European and American systems. Specific standards such as ANSI American Standard, BS British Standard, DIN German Standard, JIS Japanese Standard, etc.

1. Flat Welding Flange

Flat Welding Flange has two types, Slip-on Flange & Plate Flange.

How to lap welding the flat welding flange?

The pipe need to insert into the flange ID to an appropriate position and then lap welding.

Advantage: It is easy for welding, and the total cost is competitive, so it is widely used.

According to the internal pressure calculation, the strength of the flat welding flange is about two-thirds of the corresponding butt welding flange, and the fatigue life is about one-third of the butt welding flange. Therefore, the flat welding flange is only suitable for pipeline systems with relatively low-pressure levels and low-pressure fluctuations, vibrations and spreading.

2. Blind Flange

Blind Flanges are a form of connection, in fact, flanges with no holes in the middle.

One of its functions is to block the end of the pipeline, and the other is to facilitate the removal of debris in the pipeline during maintenance.

In terms of the plugging effect, it has the same effect as the stub end and cap. However, it is too difficult to remove the stub end, and the blind flange is fixed with bolts, which is very easy to remove.

3. Butt-Welding Flange

BW flange is also called high neck flange. It differs from flat welding flange in that there is a long and inclined high neck from the flange welding side to the flange. The wall thickness of this high neck along with the height and the direction gradually transitions to the thickness of the pipe wall, which improves the discontinuity of stress and thus increases the flange strength.

Butt-welded flanges are mainly used in places with severe working conditions, such as where the pipe undergoes thermal expansion or other loads that cause the flange to undergo greater stress or repeated stress changes; pipes with large fluctuations in pressure and temperature or high temperature, high pressure, and Sub-zero temperature pipes.

4. Thread Flange

Thread flange is the flange with thread connections. It also has FF & RF Types.

The internal hole of the flange is processed into thread and connected with the threaded pipe.

Advantage: It is convenient for installation and maintenance, it could be used on some pipelines where welding is not allowed. Such as for alloy steel flanges. As its sufficient strength, so it is not easy to weld and the welding performance also is not good. Threaded flanges can be selected.

Disadvantage: It is easy leakage if under the condition that the temperature of the pipeline changes sharply or the temperature is higher than 260 ℃ or lower than -45℃. And it is also not suitable for flammable/explosive and highly hazardous locations.

5. Socket Welded Flanges

The basic shape of the SW Flange is the same as the flat welding neck flange. The pipe need to insert into the flange ID to an appropriate position and then lap welding. Because there is a gap between the socket weld flange and the groove, it is easy to cause corrosion. If a seam is welded inside, this problem can be avoided. SW Flanges both welded on the inner and outer sides have a fatigue strength 5% greater than flat welded flanges and the same static strength.

When using SW Flange, its ID must be the same as the ID of the pipe.

The socket weld flange is only suitable for pipes with DN150 or less.

6. Lap Joint Flange (Loose Flange)

The connection of loose flange (Lap Joint Flange) is actually achieved by welding.

The difference is this type of flange is not welded with pipe directly, it uses the auxiliary component(such as stub end) welded with the pipe instead, and then the auxiliary component and the gasket are compressed by a connecting bolt. And the flange(Loose Flange) itself does not connect with the medium.

Flat welding and butt welding are both available.


1. Saving total cost. When the pipe material is special and expensive, welding the same material of the flange is expensive. You can choose the loose flange. (Flange could be a different material, only need auxiliary component material is the same as pipe material).

2. This flange could be rotated to easily align the bolt holes, and it is easy to install on large diameter pipes.

3. Suitable for places where pipes need to be disassembled frequently for cleaning or inspection. Or if you want to align the flange bolt hole, it only needs to turn the flange without rotating the pipe.


1. Withstand low pressure.

2. The strength at the welding ring is low (especially when the thickness is less than 3mm)

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